The hematopoietic stem cell (HSC) is the ultimate progenitor of all types of cells found in the peripheral blood. In addition to being capable of proliferatiion and differentiatiion into these cell types, HSC also have the capacity to proliferate without differentiation. This latter property allows a small number of transplanted HSC to repopulate a bone marrow transplant recipient. Our research interest is in how the differentiation and proliferation of HSC are controled. We have devised methods to purify HSC away from other bone marrow cells, allowing the examination of gene expression in HSC. Our most important observation is that the level of mRNA encoding retrovirus receptors in HSC is very low. Using tissue culture cell models and mouse and human bone marrow HSC, we have shown a direct correlation between the level of retrovirus receptor mRNA and the frequency with which a cell line or HSC can be transduced. Our future efforts will be devoted to identifying HSC with high levels of retrovirus receptor mRNA. Thus far we have shown that cord blood HSC which have been cryopreserved have high levels of retrovirus receptor mRNA. In mouse models we have shown that cytokine mobilization increases retrovirus receptor mRNA levels in both peripheral blood and bone marrow HSC. We are currently evaluating receptor mRNA levels in peripheral blood and bone marrow samples from cytokine treated monkeys and humans. Our plan is to evaluate retrovirus transductioon is all populations of HSC with high levels of receptor mRNA. Our ability to purify HSC has allowed us to investigate the molecular biology of stem cells. We have generated several HSC libraries from which we have extracted 3 novel genes, a tyrosine kinase transmembrane receptor, and apoptosis inhibitor, and a novel member of a family of differentiation inhibitors. The expression pattern of these genes is being evaluated in mouse and human HSC, progenitors, and mature hematopoietic cells. Our plans are to use transgenic mouse models to determine the specific function of each gene.